Existing satellite navigation systems, such as GPS (Global Positioning System), GLONASS or planned European systems (GNSS) have been realized or are planned by means of special constellations of several satellites orbiting the earth on polar paths or on geostationary paths. The planned European system GNSS, for example, could be implemented by including the satellites of the INMARSAT network moving in geostationary orbits. The satellites of the GPS move in polar orbits at an attitude of approximately 20,000 km above ground and emit radio signals, depending on the user group, on one or two frequencies in the range between 1 and 2 GHz, which transmit exact time markers to the user on the ground by means of a permanently repeated modulation with a very broadband data sequence of great length. Because of the very broadband modulation signal and the relatively low rate of its repetition, the spectrum of these radio signals gives the impression of white noise in the close vicinity of its respective center frequency. Since the phase of the radio signal is modulated, it is possible to return it to its unmodulated state in the receiver by means of negative modulation with the same modulation signal, which can be very easily reproduced because of its particular structure, and can be detected with little noise by means of a very narrow band filter.
The synchronization of the second modulating process in the receiver required for this makes possible the determination of distance differences from several satellites which are to be received simultaneously, whose respective instantaneous positions are known, and from this, analogously with the hyperbolic navigation process, the exact determination of the position of the user.
By means of the operation on two different frequencies, a compensation of the properties of the ionosphere surrounding the earth, whose structure changes in the course of a sunspot cycle during a period of approximately eleven years, as well as seasonally and daily, is offered to an appropriately equipped group of users (Lambert Wanninger "Der Einfluss der Ionosphare auf die Positionierung mit GPS" [The Effects of the Ionosphere on Positioning by Means of GPS], Hannover University, 1994, ISSN 0174-1454).
Furthermore, errors are deliberately generated by the operator (US Air Force) in order to prevent the use of commercially available receivers, for example for the rapid and easy placement of gun positions. As a result all remaining, parallel existing and planned systems with satellites in higher orbits have the main purpose of breaking the monopoly of a single system and to make available world-wide, highly exact position determinations independently of marginal political and military conditions.
The mentioned systems do not operate cooperatively, there is no bidirectional linkage between the satellites and the individual user of the systems, and therefore the number of users is infinite and, with the full operability of the system, also uncontrollable. The good viewing ability of the satellites, assured by the high orbits of the respective satellites, makes it possible to keep their number low for covering the entire surface of the earth. Twenty-one operating satellites are sufficient for GPS. However, it is hardly possible to selectively cancel the availability of the system in narrowly defined regions of the world in order to prevent their integration into the optimized operation of military systems. Furthermore, a hypothetical system operated by means of satellites in relatively high orbits with a cooperative position determination for the purpose of controlling unwanted usage, only allows a limited number of users, given the existing scarcity of frequencies.